Halogen-Driven Modulation of Structural Symmetry and Optical Properties in Hybrid Copper(I) Halides: Tunable Luminescence, Birefringence, and Composition-Dependent Second-Harmonic Generation

Abstract Organic–inorganic hybrid Cu(I)-based halides have become a promising class of optoelectronic materials due to their adjustable structures, low cost, and easy solution processing. To explore the dual regulatory effect of halide ions on structures and optical properties, two copper(I) halide compounds, namely (BuTPP)CuCl2 (1) and (BuTPP)CuBr2 (2) [BuTPP+ = (1-butyl)triphenylphosphonium, C22H24P+], were synthesized. Halogen substitution induces a symmetry transformation from non-centrosymmetric (NCS) 1 to centrosymmetric 2. The former exhibits phase-matchable second-harmonic-generation (SHG) intensity of 0.8 × KDP. Meanwhile, the birefringence is nearly doubly enhanced from 1 to 2 with halogen substitution. Moreover, halide engineering effectively tunes the luminescence color, viz., 1 emits yellow-green light, whereas 2 displays bluish-green emission. This study realizes the synchronous modulation of structural symmetry, SHG performance, birefringence, and photoluminescence via halogen substitution, which offers a powerful route to develop multifunctional optical hybrid materials.